Off-shore drilling rigs are widely used in the exploration and exploitation of hydrocarbon reservoirs under the sea floor.
The various types of off-shore drilling units include the so-call bottom-supported rigs which rest on the seafloor. Jack-up rigs are typical examples of bottom-supported rigs; they typically comprise a hull and a number of legs adapted to be lowered towards the sea floor. Such rigs may thus be towed towards their desired off-shore location with the legs in a raised position. Once the rig is at its intended position the legs are lowered and brought into contact with the sea floor. Further lowering of the legs relative to the hull causes the hull to be elevated out of the water. Many jack-up rigs have the drill floor and well centre positioned on a cantilever system that can be extended horizontally outwards relative to the hull of the rig, thus allowing the well centre to be positioned outside the periphery of the unit defined by the hull of the rig.
On an offshore bottom-supported drilling rig, at least some drilling operations are performed through a conductor pipe. The conductor pipe comprises a string of pipe installed between a position below the drill floor of a bottom-supported drilling unit, such as a jack-up rig, and the hole in the seabed. It serves as a conduit for drilling tools and the returning drilling fluid such as mud. The conductor pipe is also referred to as a riser, such as a high-pressure riser or a drive pipe or other type of riser.
To this end, a bottom-supported drilling rig comprises a blow-out-preventer (BOP) system installed below the drill floor but above the water level, and the conductor pipe extends from the BOP downwards towards the seabed. The BOP is normally arranged in a vertical tier referred to as BOP stack and installed to control downhole pressures. The conductor pipe may thus serve as a base for the BOP stack which is installed above it.
An offshore drilling rig normally comprises an access platform below the well centre and the drill floor so as to allow workers access the BOP stack. The access platform is also referred to as the Texas deck and it typically surrounds the base of the BOP stack and is suspended from the cantilever (e.g. by adjustable cables or rods) or attached to a side wall of the hull. It may e.g. be accessed from the hull of the rig by a semi-permanent stairwell.
One known type of Texas deck comprises a drive pipe support member for supporting the conductor pipe against horizontal movement. This type of Texas deck is typically attached to the hull of the rig and is also referred to as drive pipe support deck.
Bottom-supported rigs typically also comprise a tensioner system for providing vertical support to and tension in the conductor pipe also referred to as tension load. For example, the tensioner system may comprise hydraulic cylinders extending downward from the cantilever, and a tensioning frame suspended from the cylinders such that the conductor pipe is suspended in and held under tension by the tensioning frame. The tensioning frame is often also referred to as tensioning yoke. In some embodiment the tensioner system is also arranged to provide vertical (horizontal) support as well.
In many situations it may be desirable to install additional equipment below the cantilever and above the Texas deck, such as additional BOP rams, pressure control/management equipment, and/or the like. However, the available height between the Texas deck and the bottom of the cantilever may limit the space available for installation of such equipment. In particular this may be a problem in situations where the Texas deck is attached to the hull which may prevent the Texas deck from being installed further below the cantilever.
It would thus be desirable to provide a drilling rig where additional and/or larger equipment may be installed below the cantilever.
Riser tensioning systems are generally also known from other types of oil and gas platforms, such as floatable platforms. For example, U.S. Pat. No. 5,524,710 discloses an oilfield hanger and wellhead system and, more particularly, a hanger assembly for suspending a tubular string in tension from a surface wellhead on an offshore tension leg platform which floats on the water surface. In this prior art platform the BOP stack is mounted on a surface wellhead which is positioned on the rig base.
U.S. Pat. No. 3,195,639 describes a floating barge from which the riser and BOP are suspended below the surface of the water. For maintenance or repair, the BOP has to be raised to the barge.
U.S. Pat. No. 6,045,296 discloses a tensioning device for applying tension to a riser from a surface deck. A so-called Christmas tree or surface well head is shown above the surface deck from which tension is applied to the riser.
However, the above prior art systems are not directly applicable to a cantilevered bottom-supported rig where the blow-out preventer system is positioned below a cantilever and above the water surface.